This invention relates to a control method and apparatus for an elevator system with a cage driven by a motor, and more particularly to a control mechanism for preventing an elevator cage from vibrating.
U.S. Pat. No. 4,030,570 discloses an apparatus for accurately controlling the speed of an elevator drive motor by means of speed and current feedback circuits.
Although worm gears are generally used as reduction gears for elevator hoisting apparatuses, they not only have a low power transmission efficiency but also tend to needlessly consume power. As a result, the more recent trend is to use parallel axis speed reducers having helically cut gears.
Referring to FIGS. 1 through 4, a conventional elevator speed control apparatus using parallel axis reduction gears will be described. The block diagram arrangement of FIG. 1 comprises a speed command generator 1 for generating a speed command signal 1a, an adder 2 for generating an error or deviation signal 2a by collating the speed command signal 1a with an actual speed feedback signal 10a, a speed controller 3 for generating a torque command signal 3a by, inter alia, amplifying the output of the adder 2, an adder 4 for generating a deviation signal by collating the torque command signal 3a with a current feedback signal 9a, a torque controller 5 for generating an ignition signal 5a corresponding to the output of the adder 4, a thyristor converter 6 for generating a d.c. variable voltage output based on a firing angle controlled by the ignition signal 5a and comprising a pair of three-phase, full-wave, forward/inverse rectifying circuits formed by thyristors, an armature 7 of a d.c. motor controlled by the converter, a shunt field system 8 for the motor, an armature current detector 9 for generating the current feedback signal which is proportional to the motor torque, a tachometer generator 10 for generating the speed feedback signal, a speed reducer 11, an input shaft 11A formed by extending the shaft of the motor, a drive sheave 12 fixed to an output shaft 11I of the speed reducer, a main cable 13 wound on the sheave, a cage 14 coupled to one end of the cable, and a counterweight 15 coupled to the other end of the cable. The operation of the FIG. 1 system is fully conventional if not self-evident and will not be described in detail.
The sectional view of the speed reducer in FIG. 2 shows a first helical gear 11B fixed to the input shaft 11A, a parallel intermediate shaft 11C mounting a second helical gear 11D meshing with the gear 11B and a third helical gear 11E, a parallel intermediate shaft 11F mounting a fourth helical gear 11G meshing with the gear 11E and a fifth helical gear 11H, and a parallel output shaft 11I carrying a sixth helical gear 11J meshing with the gear 11F.
FIG. 3(a) shows a waveform of the speed signal 10a during the upward operation of the cage 14 without load; FIG. 3(b) a waveform of the corresponding cage acceleration; and FIG. 3(c) a waveform of the corresponding torque command signal 3a.
S.sub.1 of FIG. 3(c) represents a region wherein the motor armature 7 is power running and S.sub.2 regions wherein the armature is regeneratively running or braking. The motor torque is switched from regeneration to powering at time t.sub.1 and from powering to regeneration at time t.sub.2. At these times the engagement of the helical gears with one another in the speed reducer 11 is changed because of backlash. Consequently, the impacts shown by the arrows in FIG. 3(b) are transmitted to the cage 14 and impair the riding comfort of the passengers.
FIG. 4 shows enlarged versions of the regions at time t.sub.1 of FIG. 3, with FIG. 4(a) illustrating the speed of revolution 11Aa of the input shaft converted into that of the intermediate shaft 11C, and the speed of revolution 11Ca of the intermediate shaft; FIG. 4(b) showing the acceleration 11Ab of the input shaft, the acceleration 11Cb of the intermediate shaft, and thus the acceleration of the unloaded cage 14 when it moves upwards upon release of its friction brake due to the pull of the counterweight; FIG. 4(c) showing the waveforms of the torque command signal 3a and the motor torque T; FIG. 4(d) showing a changing engagement between the helical gears 11B and 11D during which the tooth 11Ba of the gear 11B is disengaged from the tooth 11Da of the gear 11D and contacts the next tooth 11Db.
Assuming that the cage 14 is at rest without load and starts to move upwardly, the tooth 11Ba of the helical gear 11B remains in contact with the tooth 11Da of the helical gear 11D up to time t.sub.11. Positive motor torque T is generated at this time, and when the acceleration of the input shaft 11A exceeds the acceleration of the intermediate shaft 11C the tooth 11Ba separates from the tooth 11Da. As a result, the speed of revolution 11Ca and the acceleration 11Cb of the intermediate shaft drop off. When the tooth 11Ba subsequently engages the tooth 11Db, the collision energy is determined by the relative speed difference .DELTA.v between the helical gears 11B and 11D, and when this difference is large the collision impact generates abnormal vibrations which impair passenger comfort.
These collision impacts are obviously compounded by the other helical gear pairs 11E, 11G and 11H, 11J, though reference has only been made to the gears 11B and 11D.